1. Field of the Invention
The present invention relates to liquid crystal display devices in which a first substrate having a pixel electrode and an active element and a second substrate having an opposed electrode have a liquid crystal layer interposed therebetween with each of the electrodes thereof facing each other and also relates to liquid crystal orientation methods.
2. Description of the Related Art
Conventionally, a liquid crystal display device in a TN mode in which liquid crystal material with a positive dielectric anisotropy is put in a dark state and is oriented to be in a horizontal direction relative to a substrate surface and twisted 90° between opposed substrates is widely used as a liquid crystal display (LCD) using an active matrix.
However, the TN mode has a disadvantage that it is inferior in its viewing angle characteristic and various studies have been made to improve the viewing angle characteristic thereof. As a method substituting for the TN mode, an MVA (Multi-domain Vertical Alignment) system in which liquid crystal material with a negative dielectric anisotropy is vertically oriented and liquid crystal molecules under voltage application are regulated, without giving rubbing treatment to oriented films, to tilt in directions by protrusions and slits which are provided on surfaces of the substrates has been developed. The MVA system has succeeded in improving the viewing angle characteristic to a great extent.
The structure and function of an MVA system liquid crystal display device will be described below.
The MVA system is a system which performs orientation dividing of a vertical orientation type liquid crystal by providing elements in the forms of bank-shaped (linear) protrusions and slits on the substrates. As shown in FIG. 22, FIG. 23A, and FIG. 23B (a sectional view taken along the line I–I′), the elements 103 in the forms of the linear protrusions and slits are arranged alternately on an upper substrate 101 and a lower substrate 102. Thereby, liquid crystal domains in which orientation directions on both sides of the element 103 are approximately 180° different from each other are formed in regions without the element 103 (spaced interval parts). In this way, a suitable orientation dividing is realized. This MVA system has improved the viewing angle characteristic of the liquid crystal display device to a great extent.
Here, the ‘linear (bank-shaped) protrusion’ is made of dielectric material and formed on an electrode (for example, a pixel electrode, an opposed (a common) electrode, and so on) and the ‘slit’ is a concave portion formed in a part of the electrode. Hereinafter, the same expressions in the specification of this patent application designate the above elements.
However, the conventional MVA system liquid crystal display device has a disadvantage that light transmittance of a panel is lower than that of a liquid crystal display device in a TN mode. One of the reasons will be described with reference to FIGS. 24 and 25.
FIGS. 24A and 24B show states of pixel observation when a conventional MVA panel used in general is in a white display state. FIGS. 25A and 25B show states of liquid crystal orientation.
As shown in FIG. 24A and FIG. 25B, it is seen that a line which appears dark (a dark line 105) exists in a part of a region near an edge of a pixel electrode 104. In this region, as shown in FIG. 24B (a sectional view taken along the line I–I′) and FIG. 25A, the element 103 on the pixel electrode 104 regulates liquid crystal molecules to tilt in a right direction relative to the element 103 while a slanting electric field of the edge of the pixel electrode 104 regulates the liquid crystal molecules to tilt in a left direction. Therefore, liquid crystal orientation directions defined by them are substantially opposite to each other. As a result, the liquid crystal molecules in this region are oriented in the same direction as a polarizing axial direction, which optically causes the dark line to occur and therefore, lowers the transmittance.
As shown in FIG. 26A and FIG. 26B (a sectional view taken along the line I–I′), this problem is solvable by applying a method of newly providing a bank-shaped element 106 (an auxiliary bank method) on an opposed part to the edge of the pixel electrode. The newly provided element 106 is disposed along the edge of the pixel electrode. At this time, the element 106 regulates the liquid crystal molecules to be oriented in an opposite direction to the direction defined by the slanting electric field of the edge of the pixel electrode. Thereby, the liquid crystal orientation near the edge of the pixel electrode is caused to be substantially in the same orientation direction defined by the originally provided element 103.
FIGS. 27A and 27B show the position of the dark line within the pixel at this time.
In FIG. 27A and FIG. 27B, black circles and white circles show singular points of an orientation vector and a line connecting the black circles and the white circles shows the dark line. The dark line which conventionally enters inside the pixel stays on the newly provided element 106. Here, the distribution of the singular points and the dark lines on the whole pixel is shown in FIG. 28.
In this way, the light transmittance of the panel can be improved by approximately 10% compared with that in the conventional art. Here, the newly provided element 106 works in a manner in which it helps the liquid crystal orientation approximate to the original liquid crystal orientation control defined by the originally provided element 103. Therefore, the newly provided element 106 is hereinafter called an auxiliary bank.
However, it is found that a problem of partial unevenness in brightness within the panel, which is recognized as irregular display or ununiformity in display brightness, occurs when this method is applied. After investigation, it is found that this problem is caused by the following reason.
In order to drive the liquid crystal molecules, it is necessary to form a TFT element, bus line, and pixel electrode patterns on one of the substrates. These patterns are formed by a photolithography process. At present, resist exposure is performed with the surface within the panel being divided into regions (exposure by shots using stepper machines) in order to form fine patterns of approximately several microns at the minimum with the equal shapes and width all over the panel.
At this time, overlapping widths of the substrate and a photomask sometimes deviate a little between adjacent shots from each other. This deviation causes relative position of the edge of the pixel electrode and the auxiliary bank to vary from shot to shot. As described above, the liquid crystal orientation direction defined by the edge of the pixel electrode and the liquid crystal orientation direction defined by the auxiliary bank are opposite to each other. Therefore, when the relative position of the edge of the pixel electrode and the auxiliary bank varies, orientation control balance between them varies, which sometimes influences the liquid crystal orientation near the auxiliary bank. Particularly, when deviation in overlapping width of a TFT substrate and an opposed substrate (having the auxiliary bank) is large, this problem occurs distinctly.
A difference in states of the liquid crystal orientation (the dark line) caused by the variation of the relative position of the auxiliary bank and the edge of the pixel electrode is shown in FIGS. 29A and 29B. When the overlapping width of the auxiliary bank and the edge of the pixel electrode is wide (FIG. 29A), the dark line stays on the auxiliary bank. Meanwhile, when the overlapping width is narrow (FIG. 29B), the dark line gets inside the pixel. As a result, a difference in transmittance between both of the pixels is caused. In this way, brightness among each shot is caused to be different from each other, which is recognized as irregular display or ununiformity in display brightness.
As a countermeasure for improving this problem, it can be thought of that the auxiliary bank is disposed further inside the edge of the pixel electrode than in the conventional art so that the effect of the auxiliary bank does not vary even with some degree of overlapping deviation. However, in this case, it is found that a dark region newly occurs as shown in FIG. 30 and the light transmittance of the panel is lowered.
So far, since the auxiliary bank and the bank on the pixel electrode are formed under the same condition, they also give the same influence to the orientation of the liquid crystal molecules. The bank on the pixel electrode regulates the liquid crystal molecules in the bank spaced interval part to tilt in a perpendicular direction relative to an extending direction of the bank. Here, when the auxiliary bank gets sufficiently inside the pixel electrode, the liquid crystal molecules in its vicinity also tilt in a perpendicular direction relative to an extending direction of the bank (half-tone dot meshing parts in FIG. 28). Since this direction is substantially equal to the polarizing axial direction of a polarizing plate, the light transmittance of the panel is lowered.
Furthermore, it is proposed that the auxiliary bank is made lower in height than the bank on the pixel electrode. However, this necessitates banks different in height to be formed on the same substrate and consequently a process becomes complicated.
From FIGS. 26A and 26B, and FIG. 28, it is apparent that ideally, the liquid crystal orientation near the auxiliary bank is in a direction of 45° relative to the auxiliary bank and in the perpendicular direction relative to the bank on the pixel electrode and the dark line stays on the auxiliary bank and does not get inside the pixel electrode. However, in the present structure, the various problems as described above occur and it is very difficult to stably realize the ideal orientation state.
As described above, when the MVA system is applied, the viewing angle characteristic is greatly improved. On the contrary, the slanting electric field which occurs near the edge of the pixel electrode has a big influence and promotes a so-called dark lines or a part of schlieren pattern to be formed. Even when the auxiliary bank is provided in order to cope with this problem, the influence by the deviation in mask overlapping at the time of patterning sometimes arises, and therefore, it is difficult to obtain an even liquid crystal orientation state.